The Sn2 Reaction

Sn2 Rate and Transition State

The reaction of methoxide ion with methyl bromide proceeds via
an
SN2
mechanism. It has a second order rate law:

rate = k [CH3O-] [CH3Br]

The reaction is first order with regards to methoxide ion and methyl bromide, and second order overall. This means
that the transition state must involve one molecule of methoxide and one molecule of
methyl bromide. The sim plest possible transition state is one where the methoxide
nucleophile substitutes for the bromide ion:

This is a trigonal bipyramidal transition state. The dashed lines represent forming and decomposing bonds. The
δ-
symbolizes a partial negative charge.

The transition state is formed by the flow of electrons from the nucleophilic methoxide ion into the C-O bond. At
the same time, the electrons from the C-Br bond flow onto the bromine leaving group,
weaken the bond and place a partial negative charge onto the bromine atom. All of this happens simultaneously, and
the reaction is said to be concerted. All
SN2
mechanisms are concerted.

SN2
Stereochemistry

The rate law indicates which molecules are in the transition state, but it does not specify how they come
together. This can be accomplished by attacking a stereocenter:

R1
,
R2
and
R3
represent three different groups. LG is a good Leaving Group.

The nucleophile can attack the stereocenter in two ways. In frontside attack, it attacks from the same side as the leaving group. In backside attack, it attacks from the opposite side of the leaving group. These two modes of attack give retenti
on and inversion of stereochemical configuration, respectively. Retention and inversion will yield two different stereoisomers.

The phrase "inversion of configuration" may lead you to believe that the absolute configuration must switch
after
SN2
attack. This is not always true. Recall that absolute stereochemical configuration is
determined via the Cahn-Ingold-Prelog (CIP) system, and
categorized with the labels "R" and "S." The absolute configuration inverts (from "R" to "S" or vice-versa) only
when the nucleophile and leaving group have the same CIP priority relative to the other substituents. Since good
nucleophiles and leaving gro ups tend towards equally high CIP priorities, most
SN2
reactions do
result in a switch of absolute configuration. If the nucleophile and leaving group have different relative
CIP priorities, however, the absolute configuration does not necessarily change even though inversion occurs. Keep
on your toes.

A favorite test question presents a
SN2
reaction that results in 100% retention of both
absolute and general configuration. In this case, it's likely that twoSN2
reactions take place. Two inversions yield a net retention of configuration.